1. Field of the Invention
The present invention relates to a method and a program for reducing color noise in an image signal captured by a so-called single-panel image pickup apparatus including one solid-state image sensing device, such as a charged coupled device (CCD) or one complementary metal oxide semiconductor (CMOS) device.
2. Description of the Related Art
So-called single-panel image pickup apparatuses, such as single-panel digital still cameras and single-panel digital video cameras, include one solid-state image sensing device, such as a CCD or a CMOS device. The CCD or CMOS device includes a color filter having pixels of a plurality of colors arranged in a predetermined order. In general, the image pickup apparatuses perform a plurality of processes on an image signal input from the solid-state image sensing device so as to generate an image.
For example, when the color coding of a color filter of the solid-state image sensing device is defined as a Bayer arrangement, each of the pixels has information of only one color: R (red), G (green), or B (blue). That is, the Bayer arrangement includes lines each including alternately appearing R's and G's in the horizontal direction (i.e., RGRG . . . ) and lines each including alternately appearing G's and B's in the horizontal direction (i.e., GBGB . . . ). These two types of line are alternately arranged in the vertical direction, and the numbers of G pixels, B pixels, and R pixels are in a ratio of 2:1:1.
Three signals for the primary colors R, G, and B are necessary for each of the pixels of an image. Accordingly, missing color information (missing color signals) for each pixel is generated by interpolating pixels surrounding the pixel and generates the color information for the R, G, and B colors for each pixel. Such a process for generating three R, G, and B color information items (three primary color signals) for each pixel using raw image data (original data acquired from the image sensing device) having information for only one color is referred to as a “demosaic process (synchronization process)”.
After a demosaic process is performed, an R signal, a G signal, and a B signal for each of the pixels are Y/C-converted into a luminance signal (Y) and a color difference signals (Cb/Cr). Thereafter, an image to be displayed is generated from an image signal including a luminance signal (Y) and a color difference signals (Cb/Cr). Alternatively, an image signal including a luminance signal (Y) and a color difference signals (Cb/Cr) is record on a recording medium.
In recent years, the pixel size of solid-state image sensing devices has been reduced. Accordingly, the S/N ratio (a signal to noise ratio) has been degraded, and therefore, noise in an image has become more of a problem. Noise in a color image includes luminance noise, which is noise on a luminance signal component, and color noise, which is noise on a color difference signal component. By reducing color noise, the quality of an image can be increased.
For example, for digital still cameras, Japanese Unexamined Patent Application Publication No. 2005-311455 describes a technique for efficiently removing noise from a raw image data by selecting a pixel area appropriate for a noise reduction process and performing the noise reduction process using the selected surrounding pixels.
In addition, Japanese Unexamined Patent Application Publication No. 2004-221838 describes a technique for accurately reconstructing an edge in a demosaic process by interpolating the pixel value of a pixel of interest using the pixel values of pixels in a predetermined area extending along a direction in which the edge extends.
However, if color noise is removed from raw image data, a luminance signal component is affected, and therefore, the luminance value of each color signal may be changed. As a result, an edge portion may be displayed with a false color, and the quality of the image may be significantly deteriorated.
Therefore, since a color difference signal for representing a color is generated after Y (luminance)/C (color) conversion is performed, a color noise reduction process is performed after the Y/C conversion is performed, in general. However, although color noise has an impulse shape before a demosaic process is performed, the characteristic of the noise is changed after the demosaic process is performed (i.e., the shape of the noise is changed to a shape other than an impulse shape). This is because the color noise signal passes through complicated filters for an interpolation sub-process and an aperture correction sub-process of the demosaic process. If the characteristic of the noise signal is changed, it is difficult to separate a noise signal component from a picture signal component. Accordingly, it is difficult to completely remove the color noise.